Cycles SSS not working on AMD Radeon GPU
The fact that a feature is supported in Cycles doesn't mean it is supported for all computing methods, nor across all devices.

It is largely known that OpenCl computing on AMD branded cards is not at the same level of support as CPU rendering or CUDA on Nvidia cards. Not only is performance non-optimal, there a lots of bugs and limitations, and many features are not yet supported.

[According to the manual as of 2.78# series SSS is not supported in AMD cards as stated in the Render Features page.

That may change in future, Mai Lavelle's work on Cycles Split Kernal may make it to 2.79, and may bring new features with it including SSS for OpenCL